Before her death, friends visiting Baroness Thatcher at her grand house in Chester Square, Belgravia, would find the living room dominated by a large portrait of Thatcher herself, below which sat a silver bowl marking her 10th anniversary as prime minister. On either side on the mantelpiece, next to a photograph of her late husband Denis, pride of place was taken by framed portraits of her two American-born grandchildren, Michael and Amanda.

Lady Thatcher's "greatest delight", she told an interviewer in the late 90s, was "when my daughter-in-law sends me photographs of the grandchildren. Apart from seeing them in the flesh, that is the greatest pleasure I have in the whole year, far exceeding everything else."

It is fitting, therefore, that however grand Lady Thatcher's funeral , her funeral procession will be led by the siblings. And just as Michael did a decade ago as a 14-year-old boy at the funeral of his grandfather Sir Denis, Amanda Thatcher, now 19, will read one of two lessons, the other delivered by David Cameron. For a young, discreet Texan college student, it will be an abrupt entry into the global spotlight.

Despite — or very possibly because of — the chaotic notoriety of their father Sir Mark, now divorced from their mother Diane, Michael and Amanda are described by friends as modest, unshowy young people, both of whom, under Diane's influence, are dedicated evangelical Christians, and whose social and political conservatism met with their grandmother's vocal approval.

Amanda Thatcher is now studying at the University of Richmond in Virginia; her school reports show was a talented sportswoman, excelling in athletics, and who was voted "most likely to change the world" by her high school peers.

Like his grandmother, Michael studied chemistry at college and now works in a pharmacy in the wealthy Dallas suburb where the siblings still live with their mother and her second husband. A talented running back for his school's American football team, he recently served on the board of VOCES, a pro-Republican body offering what it calls "a voice for conservative Hispanics".

"Michael Thatcher has always been so discreet and prudent about his relationship with Lady Thatcher," the organisation's director, Adryana Boyne, wrote this week, calling him and his sister "humble and kind".

But if Amanda and Michael have made an effort to avoid media attention as young adults, that luxury was not always possible during their parents' tumultuous marriage. Though Mark Thatcher's union with the Texan millionairess Diane Burgdorf lasted almost two decades before finally ending in divorce, the marriage was under strain almost as soon as it had begun.

In an extraordinarily bitter broadside in 2006, when her daughter was just 13, Burgdorf wrote in a British newspaper about how her husband's repeated infidelity had led her to hire private detectives to spy on him, and to delay having a second child because of doubts over whether their marriage would survive.

Though they lived a life of some luxury, with a butler and nanny and homes in London, Dallas and Switzerland, she described Mark as a distant and harsh father whose favoured expression about his children was: "Those things come with nannies."

By 2004, after a move to Cape Town, when Mark was arrested for his involvement in an attempted coup in Equatorial Guinea, the marriage had finally broken down, devastating Amanda in particular, according to her mother. Diane arranged to bring the children, who have dual British-US citizenship, back to Texas to be close to her own parents.

Despite the hurdles caused by distance, infirmity and family breakdown, Diane was keen to preserve Thatcher's relationship with her grandchildren. The former prime minister and Sir Denis spent most Christmases with Mark's family in Dallas or Cape Town, and after her husband died and she became too unwell to travel, Lady Thatcher spoke to her grandchildren on the phone frequently. Even after her marriage broke down, Diane maintained a close relationship with her former mother-in-law, who would ring to check on the children. She, too, will attend the funeral.

The children, by all accounts, may have thrived on their return to Texas, but the move also cut them off from their father, whose conviction over the coup attempt bars him from entering the US. When aged 12, the Sunday Times reported, Amanda wrote to President Bush asking him to intervene. "You know how you feel about your daughters? I want my Daddy back in America."